Optical applications commonly require the sensing and control of the wavelength of a light source. This is a particular concern within the optical telecommunications industry, in which light from several optical sources may be transmitted along a common optical fibre at different, closely spaced wavelengths. Such applications require optical sources producing light with a high level of wavelength stability and a narrow bandwidth.
Lasers, such as semiconductor lasers, provide narrow bandwidth emission, but can drift and jump in wavelength. Accordingly, in applications in which wavelength stability is required, a wavelength locker is commonly employed, in which an optical wavelength sensor is used to monitor the emission wavelength from the optical source and an electrical feedback system provides control of the optical source in correspondence with the monitoring.
Wavelength lockers that are commonly deployed in the optical telecommunications industry typically comprise an arrangement of bulk optical components. Examples of such systems are described in U.S. Pat. No. 7,161,725 and U.S. Pat. No. 5,825,792. Such arrangements have a sizable footprint inside a compact optical telecommunications package. Further, such lockers are expensive to manufacture, not least due to the precision require in package assembly in order to accurately align each optical element.
A monolithically integrated wavelength sensor suitable for use in a wavelength locker is a semiconductor Mach-Zehnder interferometer (MZI), in which light is split between two arms of an interferometer that have different optical path lengths and is then recombined, producing constructive or destructive interference between the light from the two arms, which is a function of the frequency of the light. However, disadvantageously the performance of the semiconductor MZI varies as a function of temperature. Attempting to overcome the temperature dependence of semiconductor MZIs has previously required the use of complex structures that are difficult to manufacture, such as those discussed in “A temperature insensitive InGaAsP—InP optical filter” by H. Tanobe et al., IEEE Photonics Technology Letters, Vol 8, No 11, 1996. Further, manufacturing yield is vulnerable to the manufacturing tolerances of the optical splitter and recombiner.
Thus a need remains in the industry for an alternative design of interferometer which seeks to mitigate at least some of the disadvantages of prior art interferometer designs.